I found that the (0x5) error noted in my original post above is indicative of a code signing problem. So I deleted the gdb certificate, recreated the gdb certificate (had to create a Login certificate, export it then import as a System certificate) then code signed the gdb binary (not the link at usr/local/bin/gdb. I originally signed this link and, re-checking myself steps, found that I had to sign the gdb binary).

codesign -fs gdb-cert /usr/local/Cellar/gdb/8.1/bin/gdb

Note that I used the "f" option to force sign the binary file because it had already been code signed by my previous installation attempt.

Then launch Lazarus (with Debugger_Startup_Options set to startup-with-shell off. I still get the same error message as enemy original post. (See attachment) I checked the 3 troubleshooting conditions in the GDB install wiki and all should be OK.

Note the location of the gdb binary on my (High Sierra) computer: /usr/local/Cellar/gdb/8.1/bin. Is this the location that others have for the binary? Have I still not signed the correct gdb file?